1. Field of the Invention
The invention relates to a method for driving a pixel of a ferroelectric liquid crystal cell having a distorted helix structure (DHF-LCD). DHF-LCDs are described in European Patent EP 0 309 774 B1.
2. Description
Ferroelectric liquid crystal cells having a distorted helix structure can be operated in two different modes. In the asymmetrical mode the cell is disposed between crossed polarisers such that the transmission is at a minimum for a certain voltage, for example negative voltage -U.sub.o, and at a maximum for a certain voltage, for example positive voltage +U.sub.o. In the symmetrical mode the cell is disposed such that the transmission for 0 V applied voltage is at a minimum-and increases for positive and negative voltages.
In the asymmetrical mode the cell is more sensitive, that is the electro-optical effect is approximately twice that in the symmetrical mode. Against this there is the risk that, in the event of the driving not being free from a DC voltage or the spontaneous polarization having the same polarity for many frame times, electrochemical processes are initiated or polarization charges are generated in the orientation layers of the liquid crystal cell. Both effects may lead to phantom images. In the symmetrical mode, this can be avoided by driving an image alternately with a positive voltage and a negative voltage.
In both modes, it is important to be able to switch as quickly as possible from one grey scale value to another. In the symmetrical mode, this is more difficult because a greater liquid crystal movement is required for the same change of grey scale value.
Important applications of DHF-LCDs require active matrix driving, that is, semiconductor elements (transistors or diodes) are associated with each pixel and permit multiplex display operation.